1. Field of the Invention
This invention relates to microfabricated, gas-filled radiation detector assemblies, methods of making and using same and interface circuit for use therewith.
2. Background Art
The following references are noted herein:    [1] S. A. Audet et al., “High-Purity Silicon Soft X-Ray Imaging Sensor,” SENSORS AND ACTUATORS, (A22) Nos. 1–3, March 1990, pp. 482–486.    [2] R. Wunstorf, “Radiation Hardness of Silicon Detectors: Current Status,” IEEE TRANSACTIONS ON NUCLEAR SCIENCE, (44) Nos. 3, June 1997, pp. 806–14.    [3] E. Bertolucci et al., “BETAview: A Digital β-Imaging System for Dynamic Studies of Biological Phenomena,” NUCL. INST. AND METHODS, A381, 1996, pp. 527–530.    [4] W. J. Price, “Nuclear Radiation Detection,” McGraw-Hill, 1964, p. 123.    [5] G. Charpak et al., “Micromegas, A Multipurpose Gaseous Detector,” NUCL. INST. AND METHODS, A478, 2002, pp. 26–36.    [6] G. F. Knoll, “Radiation Detection and Measurement,” JOHN WILEY AND SONS, 2000, pp. 161, 180.    [7] C. G. Wilson et al., “Profiling and Modeling of DC Nitrogen Microplasmas,” JOUR. OF APPLIED PHYSICS, 94(5) September 2003, pp. 2845–51.
Radioactive materials, particularly uranium, are stockpiled in large and small quantities all over the world with varying degrees of security. There is a perpetual risk of their potential use in “dirty bombs,” which use conventional explosives to disperse dangerous radioactive materials. Uranium-238 naturally decays into 234Th and then 234mPa, emitting 0.8 MeV beta particles, which are essentially high energy electrons. Other possible dirty bomb ingredients include a number of beta sources, including 90Sr and 204Tl. The former is a particularly hazardous material, as it is easily absorbed into the human body, where it displaces calcium in bone, remaining there with a radioactive half-life of 27 years. Thus, there is considerable motivation to develop miniaturized sensors for radioactive materials.
Since a few radioactive materials emit X-rays, one possibility is to exploit X-ray detectors, which have benefitted from solid-state technology in recent years [1]. Unfortunately, most radioisotopes are not sources of X-rays, and the best way to detect most of the target species for dirty bombs is through their emission of beta particles. Solid-state detectors for beta particles exist, but they are relatively large, with sizes on the order of 1 cm2. They typically require cryogenic cooling to distinguish radiation type and energy, and are particularly subject to radiation damage [2]. Another type of device uses pixelated silicon structures at room temperature to provide spatial imaging of beta particle flux [3].
Geiger counters, however, are the preferred sensors for detecting beta radiation [4]. As shown in FIG. 1, typical Geiger counters utilize a tube 10 held under vacuum, with a rod-like anode 12 and concentric cathode 14. The tube 10 is biased at 500–1000 volts, and a thin window—typically mica—allows passage of beta radiation. This radiation ionizes the gas to form ions 18 and electrons 19 at some statistical rate, resulting in an avalanche breakdown, which is measured by circuitry as a “count” corresponding to one event. These gas-based detectors are very reliable, temperature insensitive, require only simple circuitry, and measure over a much wider range of radiation species and energies. Miniaturized gas-based detectors exist, but work only for detection of photon-based radiation, such as X-rays [5]. Again, as very few isotopes emit X-rays, there is a compelling need for a miniaturized beta radiation detector.
Macroscale gas-based devices have been widely used in the field of radiation detection [6]. Virtually all gas-based detectors rely on the impinging radiation ionizing the fill gas, with the resulting electrons accelerated by an electric field, ionizing more neutral species, thereby creating an avalanche breakdown. The general form of the electron density in a cascade of length x is given by:n(x)=n(0)exp(αx)  (1)Here, α is the first Townsend coefficient of the gas, a function of the gases ionizability, and electron capture cross-section.
Typical detectors fall into four regimes of operation, defined by the applied electric field, electrode geometry, and the pressure and species of fill gas (FIG. 3). The four regimes all have electric discharges with differing physical properties. (These regimes apply to both beta particles and photons like X-rays and gamma particles). The regime with the lowest voltage is the ion saturation region, where the only charge collected is by gas directly ionized by impinging radiation. As the voltage across the device is increased, avalanche breakdown begins to occur, and the amount of collected current increases. This is the proportional region: the amount of current is roughly proportional to the energy of impinging X-rays or gamma particles, as photon radiation is completely absorbed by the background gas. In contrast, impinging beta particles impart only a portion of their kinetic energy to the ionization of gas, so the resulting current created is not correlated to the beta energy; the proportional region is more limited for beta particles.
As the voltage is increased, the dependence of the current pulse upon the energy of the radiation is diminished even for photon radiation. This non-linearity is primarily due to the difference in mobilities between ions and electrons. In the limited proportionality regime, the much slower ions are sufficient in quantity to create a space-charge region which distorts the local electric fields. This limits the total charge, such as created by avalanching and is dependent on the electric fields. (Similar space-charge regions have been found to be the reason for lower charge densities in previously reported microplasmas [6]). As the voltage is increased further, the impinging radiation generates a self-sustaining discharge; this is the Geiger-Muller region. The total amount of current collected in this region for a cylindrical proportional counter is given by:
                              ln          ⁢                                          ⁢          M                =                                            V                              ln                ⁡                                  (                                      b                    /                    a                                    )                                                      ·                                          ln                ⁢                                                                  ⁢                2                                            Δ                ⁢                                                                  ⁢                V                                              ⁢                      (                                          ln                ⁢                                  V                                                            pa                      ⁢                                                                                          ⁢                                              ln                        ⁡                                                  (                                                      b                            /                            a                                                    )                                                                                      ⁢                                                                                                                            -                              ln                ⁢                                                                  ⁢                K                                      )                                              (        2        )            Here, M is the multiplication factor, the quantity of electrons from a single incident. V is the applied voltage, a and b are the anode and cathode radii, respectively; p is the ambient gas pressure, and ΔV and K are constants of the background gas, related to electron mean free path and ionizability.